Sensorless driving method for brushless DC motor

Abstract

A sensorless driving method for a brushless DC motor is provided. The time for the motor to rotate an electrical angle 60° is obtained by alternatively counting the occurrences of zero crossings with two counters and comparing the counted values, and the motor is delayed an electrical angle of 30°, by which a precise commutating time is obtained. The driving method provides a mask-based phase shift digital detection mechanism for effectively detecting true zero-crossing points. The driving method further provides an inhabitation mechanism with the function of soft-switch for inhibiting noise caused by transistor switching. By using these two counters, the time for the motor to rotate two electrical angles 30°−Δθ and 30°+Δθ are obtained and stored in two registers. The time period before and after the commutating point is added into a pulse width modulation (PWM) signal to reduce the noise and vibration.

1. A sensorless driving method for a brushless DC motor, the method comprising:
 detecting zero-crossings by a mask-based phase shift digital detection mechanism for detecting zero-crossings; and 
 inhibiting electronic noises caused by transistor switching by an inhibition mechanism; 
 wherein a first counter and a second counter included in the mask-based phase shift digital detection mechanism are used for calculating commutating points of the brushless DC motor, wherein when a first zero-crossing occurs, the first counter starts to count until a next zero-crossing occurs; and then the second counter starts to count, while a value counted by the first counter is divided by 2 and a result is stored in the first counter; foregoing steps are performed repeatedly, 
 wherein a commutating point is obtained when a value counted by the second counter reaches the value stored in the first counter; similarly, another commutating point is obtained when the value counted by the first counter reaches the value stored in the second counter. 
 
   
   
     2. The sensorless driving method according to  claim 1 , wherein the inhibition mechanism for inhibiting electronic noises caused by transistor switching is a soft-switch method for reducing noises by controlling the brushless DC motor through pulse width modulation (PWM), wherein the soft-switch method is to reduce a PWM duty cycle within an electrical angle Δθ before and after the commutating point, so as to reduce various routing voltages of the brushless DC motor. 
   
   
     3. The sensorless driving method according to  claim 2 , wherein the soft-switch method for reducing noises is to reduce the PWM duty cycle to or below 20%. 
   
   
     4. The sensorless driving method according to  claim 2 , wherein according to the soft-switch method for reducing noises, a first register and a second register are used for detecting whether the electrical angle is within Δθ before and after the commutating point, wherein the first register is used for recording a position of the electrical angle Δθ before the commutating point, and the second register is used for recording the position of the electrical angle Δθ after the commutating point, wherein
 when the second counter starts to count, a content of the first register is the value stored in the first counter minus the result of the stored value divided by 30, that is, the value stored in the first counter is a position value of the electrical angle Δθ before the commutating point, and a content of the second register is the value stored in the first counter plus the result of the stored value divided by 30, that is, the value stored in the second counter is the position value of the electrical angle Δθ after the commutating point; while when the first counter starts to count, the contents of the first and the second registers are calculated with the value stored in the second counter. 
 
   
   
     5. The sensorless driving method according to  claim 2 , wherein according to the soft-switch method for reducing noise, and a first register and a second register are used for detecting whether the electrical angle is within Δθ before and after the commutating point and whether the electrical angle is within Δθ before and after the commutating point is detected by comparing values counted by the first counter and the second counter and values stored in the registers; the PWM duty cycle is reduced if the value counted by the first counter or the second reaches the value in the first register, and the original PWM duty cycle is restored when the value counted by the counter reaches the value in the second register.